1. Field of the Invention
This invention relates to a process for treating a kaolin clay contaminated with iron-containing minerals such as chlorite and siderite to improve its brightness and, more particularly, to an improved process of bleaching a Minnesota primary kaolin clay with a novel combination of a strong acid leach and a magnetic separation.
2. Description of Related Art
Extensive deposits of a primary kaolin in the Minnesota River Valley region of the United States have been known for many years. These deposits have been evaluated on numerous occasions over the past fifty years in attempts to produce a kaolin suitable for paper coating and paper filling applications. Among the agencies that have been involved in unsuccessful attempts to commercialize Minnesota kaolin have been the Legislative Commission on Minnesota Resources (LCMR), the Minnesota Department of Natural Resources, the Minnesota Geological Survey, Natural Resources Research Institute, and the Mineral Resources Research Center. At least one of the major kaolin producers in Georgia has worked with these clays and holds extensive mineral rights in the area. Notwithstanding these efforts, thus far no one has identified a suitable process for producing an 80-87 brightness kaolin as measured by the standard G.E. scale (see TAPPI Procedure T-646-05-75 and T-649-05-75) on a meaningful portion of these reserves. As those skilled in this art appreciate (e.g., see U.S. Pat. No. 4,997,550), even brightness improvement of less than a full point can be very significant.
Among the beneficiation methods previously tried on Minnesota kaolin are the following: degritting and fractionation followed by steps such as high gradient magnetic separation (HGMS), flotation, delamination and leaching with sodium dithionite (hydrosulphite), normally under an acid condition. In most cases, the degritted brightness of the Minnesota kaolin is in the 60-70 brightness range and even magnetic separation rarely furnishes more than 5 brightness points improvement. Sodium dithionite leaching raises the brightness a maximum of 10 points, but the resulting low brightness kaolin remains below about 80 brightness nonetheless, and does not consistently meet kaolin paper filler standards.
U.S. Pat. No. 3,193,344 describes a process for bleaching kaolin clay contaminated with highly colored iron impurities using the combination of a reducing agent and an iron binding agent generally described as a water-soluble polyfunctional organic compound, one active group of which comprises a mercaptan (--SH) radical and another active group of which comprises a radical, which together with the mercaptan group is capable of chelating iron. According to the patent, the clay first is treated with the reducing agent to solubilize iron as ferrous iron. Preferably, the reaction of the clay and the reducing agent is conducted at a slight degree of acidity. Water-soluble salts of hydrosulphurous acid are preferably employed as the reducing agent. Salts identified as particularly satisfactory for this purpose include sodium hydrosulphite, zinc hydrosulphite, and calcium hydrosulphite. Further, hydrosulphurous compounds, such as taught in U.S. Pat. No. 2,339,594 also are taught; e.g., alkali metal and zinc salts of hydrosulphurous acids and the sulfoxylates.
Thereafter, the reduced clay is treated with the iron binding agent. Preferably such iron binding agent is a water-soluble acid containing a carboxyl and a mercaptan group, preferably with the --SH group in the alpha position relative to the carboxyl or group, such as in mercaptoacetic acid. Such a mercapto-carboxy acid forms in aqueous solution a water-soluble, or at least a water-dispersible, compound or complex of sufficiently slight degree of dissociation to prevent reaction of iron and oxygen in a reaction mixture and thus so prevents oxidation and reabsorption of iron on the clay from which it was previously removed. The complexed iron then is readily removed from the clay, as by filtration. Other compounds which similarly form water-soluble compounds with ferrous iron of a sufficiently low degree of dissociation to prevent reoxidation of ferrous iron compounds to ferric iron compounds in presence of dissolved oxygen, water, and clay include thiomalic acid and mercaptoethanol. For convenience, the clay can be treated with a mixture of the reducing agent and the iron binding agent. This admixture of such dibasic mercaptan acid and reducing sulfoxylate is effective at the relatively acid pH conditions at which such sulfoxylate is most effective to reduce iron compounds associated with even relatively acid reacting clays without producing an objectionable or strong odor.
U.S. Pat. Nos. 5,061,461 and 5,085,707 also describe techniques used for processing kaolin clay, particularly clay used for paper tiller and coatings. In summarizing conventional processing procedures, it is noted that following initial particle sizing, the clay is leached to remove iron-based color compounds by first acidifying the clay with sulfuric acid or alum to a pH of 3 to 5 to solubilize the iron and flocculate the clay, which aids subsequent filtration, followed by treatment with a reducing bleach agent such as sodium hydrosulphite (dithionite) which generally is more effective at an acid pH. This bleaching, typically conducted at a kaolin slurry solids content of about 30 to 40%, reduces the iron to the more soluble ferrous form. The iron then is removed via a dewatering process (filtration). The clay is flocculated and then dewatered to a solids level of about 60%. The filter cake then either is dried or redispersed with additional clay if it is to be sold as about a 70% solids slurry. The '707 patent indicates that to produce high brightness products, the clay may be processed through flotation or magnetic separation. As noted in the '461 patent this normally takes place before dithionite leaching and filtration. See also U.S. Pat. No. 3,853,983 in this regard, which describes a process for improving the brightness of an iron pyrite-contaminated clay using a combination of magnetic separation and dithionite leaching.
Other patents that relate to the beneficiation and brightening of kaolin clays include U.S. Pat. Nos. 3,826,365; 3,853,983; 4,055,485; 4,227,920; 4,419,228; and 4,781,298.